In the field of free radical polymerization of vinyl monomers it is known that a wide range of temperatures may be employed, for example, from about -60.degree. to over 100.degree. C. When the polymerization temperature is decreased usually the molecular weight, and therefore, the melt viscosity increases which adversely affects the processability of the polymer, particularly for such applications as bottle making, film making, extrusion, and the like.
It has been proposed to employ chain transfer agents in the polymerization reaction medium. However, while most known chain transfer agents are useful for lowering the molecular weight of the polymers, they have an adverse effect on the heat stability of the polymer, such as polyvinyl chloride, which, in turn, has a bad effect on the processability of the polymer.
Thus it can be seen that just employing a lower temperature during polymerization does not produce the desired processable polymer for many applications. Further, when operating at very low temperatures (below 0.degree. C.) costly and hard to control refrigeration equipment is needed.
Kinetic models for the polymerization of vinyl chloride, in bulk or aqueous suspension systems, show a two phase reaction, wherein part of the polymer is formed in solution in vinyl chloride while a part is formed in polymer swollen with vinyl chloride. The reaction rate in the polymer phase is faster than that in the monomer phase, so as more polymer is formed the overall polymerization rate increases and a progressively higher fraction is polymerized in the polymer phase. Examination of the polymer produced early in the reaction, and highest in solution polymer content, reveals that the polymer has an impaired heat stability. If the reaction is allowed to proceed to around 95% conversion, again, an impaired heat stability is observed, due to polymerization under monomer starved conditions. As pointed out, in these polymerization systems water is the suspension agent and the polymer produced is insoluble in both the water and the monomer.